Firefox 27.0 is rather old and vulnerable as it was released way back on [https://website-archive.mozilla.org/www.mozilla.org/firefox_releasenotes/en-us/firefox/27.0/releasenotes/ February 4, 2014] Perhaps they meant Firefox 127.0 or later? as Fx 127.0 was released June 11, 2024. Though you appear to have macOS 10.13 so the most current version you can run is Firefox 115 ESR currently at 115.3.0esr. Firefox 116.0 to current requires macOS 10.15 (ten.fifteen) or later to run.